This invention relates to the preparation of phosphorus oxyacids, and more particularly to novel processes for the preparation of oxyacids by catalytic reaction of water and elemental phosphorus.
Oxyacids of phosphorus are important precursors for the synthesis of other phosphorus species having various applications, for example, in herbicides, insecticides, fertilizers, flame retardants and plasticizers.
Phosphoric acid for use in fertilizer manufacture is conventionally prepared by acidulation of phosphate rock with sulfuric acid, resulting in substantial generation of by-product gypsum or calcium sulfate hemihydrate which must be disposed of either as a by-product or waste material.
Environmental and corrosion issues may also arise from the generation of HF by acidulation of fluoride contained within phosphate rock.
Higher purity phosphoric acid is produced by oxidation of elemental phosphorus to phosphorus pentoxide, and absorption of phosphorus pentoxide in dilute phosphoric acid. This process requires a combustion furnace in which phosphorus is burned to phosphorus pentoxide at temperatures in excess of 3500xc2x0 F., and is generally adapted for the production of phosphoric acid only on a large scale.
Phosphorous acid has been conventionally manufactured by hydrolyzing a halogen derivative of phosphorus, such as phosphorus trichloride. Since the halogen derivatives are prepared from elemental phosphorus, an economic advantage could be realized by preparing phosphorous acid directly from elemental phosphorus. Direct preparation could also provide environmental benefits by avoiding the use of halogen-containing phosphorus starting materials and production of halogen-containing by-products.
As described by Engel, xe2x80x9cOxidation of Hypophosphorous is Acid by Hydrogenated Palladium in the Absence of Oxygen,xe2x80x9d Compt. Rend. Acad. Sci., 1890, pp. 786-787, phosphorous acid can also be prepared by oxidation of hypophosphorous acid with water in the presence of a palladium catalyst.
However, commercial processes are not readily available for the economical preparation of the hypophosphorous acid starting material without formation of phosphine or other undesirable by-products.
Christomanos (Z. Anorg. Chem., 41, 305-14, 1904) describes an analytical procedure for determination of elemental phosphorus in organic solutions by a metal induced disproportionation to phosphorous acid and a Cu phosphide:
P4+CuSO4+6 H2Oxe2x86x92Cu3P2+3H2SO4+2H3PO3
Comparable reactions of elemental phosphorus with Cu2NO3 are also disclosed. Only stoichiometric reactions are described. Atmospheric oxygen is said to have an oxidizing function. After four hours, Cu phosphide disappears and the solution contains only Cu phosphate.
White phosphorus, the elemental phosphorus allotrope also referred to as yellow phosphorus or tetraphosphorus (P4), is a potential starting point for the synthesis of a variety of phosphorus species. The tetrahedral structure of white phosphorus contains six phosphorus-phosphorus bonds and can provide a large number of reactive species having an intermediate existence in phosphorus reactions. As noted, tetraphosphorus is the raw material for one of the major commercial processes for the manufacture of phosphoric acid. If tetraphosphorus could be used as a starting material for the manufacture of other oxyacids of phosphorus without intermediate halogenation, significant economic advantages might be realized, especially if the reaction could be conducted under relatively mild conditions. However, in the exothermic reaction of phosphorus with oxygen, it is difficult to control the reaction short of the formation of the P(V) oxide, i.e., the anhydride of phosphoric acid.
Ipatiev U.S. Pat. Nos. 1,848,295 and 1,895,329 describe processes for the preparation of phosphoric acid by catalytic oxidation of liquid phosphorus with water at high temperature and pressure. Catalysts include salts of copper and nickel, copper or nickel phosphide being formed in the reaction. Ipatiev reports that phosphorous acid is formed as an undesired by-product of the oxidation reaction, particularly early in the reaction, but does not disclose the fraction of phosphorous acid present in the reaction mixture, or the relative proportions of phosphorous and phosphoric acid present, at any time during the reaction.
Ipatiev teaches that the reaction is preferably conducted at temperatures of 300xc2x0 C. or above, but the ""329 patent includes an example at 200xc2x0 C. in which by-product copper phosphide is found in the phosphorus phase at the end of the reaction.
Numerous references describe the preparation of phosphoric acid by catalytic vapor phase oxidation of phosphorus with water at temperatures above 600xc2x0 C., commonly above 1000xc2x0 C. Various catalysts are disclosed for use in these reactions, including copper, silver and a wide variety of other metals, particularly other Group IB and Group VIII metals, certain Group VI metals (e.g., Cr, Mo, W and U), certain Group VII metals (e.g., Mn), and/or their oxides, salts and/or phosphides. To prevent leaching of catalyst out of the reaction zone, it has been proposed to use various supports for active catalysts, including, for example, pyrophosphates of Ti or Zr. Liljenroth U.S. Pat. No. 1,605,960, e.g., also lists noble metals such as Ru, Rh, Pd, Os, Ir or Pt as catalysts for the reaction.
Among the several objects of the present invention may be noted the provision of an improved process for the manufacture of oxyacids of phosphorus; the provision of such a process which can be controlled to produce lower oxyacids of phosphorus, especially phosphorous acid; the provision of such a process which can be controlled to produce phosphorous acid in high selectivity; the provision of such a process which can be controlled to produce phosphorous acid in high yield; the provision of such a process which can be operated to produce phosphorous acid in reasonably high concentration; the provision of such a process which can be operated with minimal environmental emissions; and the provision of such a process which does not use halogen-bearing raw materials or produce halogenated by-products.
Briefly, therefore, the present invention is directed to a process for the preparation of an oxyacid of phosphorus comprising oxidizing elemental phosphorus by catalytic reaction with water at a temperature below 200xc2x0 C.
The invention is further directed to a process for the preparation of phosphorous acid comprising catalytically oxidizing elemental phosphorus by reaction with water under conditions effective to produce an oxidation reaction mixture comprising a lower phosphorus oxidation product, the ratio of the sum of the concentrations of P(I) and P(III) species to the concentration of P(V) species in said reaction mixture being least about five.
The invention is also directed to a process for the preparation of an oxyacid of phosphorus comprising contacting condensed phase elemental phosphorus with water at pressure below about 20 atm in the presence of a catalyst for the oxidation of phosphorus by reaction with water. Elemental phosphorus is catalytically oxidized by reaction with water.
The invention is also directed to a process for the preparation of an oxyacid of phosphorus comprising oxidizing elemental phosphorus by catalytic reaction with water in a catalytic reaction zone to produce a lower phosphorus oxidation product at a rate of at least 0.01 kg/hr per unit volume of said reaction zone as expressed in m3.
The invention is also directed to a process for the preparation of an oxyacid of phosphorus comprising catalytically oxidizing elemental phosphorus in a continuous catalytic reaction zone to produce a lower phosphorus oxidation at a rate of at least 1xc3x9710xe2x88x927 kg/hr-g catalyst.
The invention is further directed to a process for the preparation of phosphorous acid comprising catalytically oxidizing elemental phosphorus with water, thereby producing an aqueous reaction mixture comprising at least 5% by weight of a lower phosphorus oxidation product, the reaction between phosphorus and water being conducted in a heterogeneous reaction system comprising a water phase and a condensed phase comprising elemental phosphorus, said condensed phase containing a catalyst for the reaction.
The invention is also directed to a process for the preparation of an oxyacid of phosphorus comprising contacting a condensed phase comprising elemental phosphorus with an aqueous phase in the presence of a catalyst for the oxidation of phosphorus by reaction with water, active sites of the catalyst being maintained in contact with the condensed phase comprising phosphorus preferentially to said aqueous phase during the catalytic oxidation reaction.
The invention is further directed to a process for the preparation of an oxyacid of phosphorus comprising contacting a condensed phase comprising elemental phosphorus with an aqueous phase in the presence of a catalyst for the oxidation of phosphorus by reaction with water, the catalytic oxidation reaction occurring preferentially in said elemental phosphorus phase.
The invention is further directed to an apparatus for oxidation of elemental phosphorus to oxyacids of phosphorus.
The apparatus comprises a liquid/liquid contact zone for contacting an aqueous phase reagent with a substantially water-immiscible condensed phase comprising tetraphosphorus; and a catalytic reaction zone for contacting the water-immiscible condensed phase with a catalyst for the oxidation of elemental phosphorus by reaction with water.
The invention is particularly directed to an apparatus comprising a reservoir for a body of a substantially water-immiscible liquid containing elemental phosphorus, and means for introducing an aqueous liquid into the reservoir for flowing across a surface of the body of liquid containing elemental phosphorus. As the aqueous phase flows across the surface of the body of the water-immiscible liquid, water may be transferred from the aqueous phase to the phase comprising elemental phosphorus, and phosphorus oxidation products may be transferred from the phase comprising elemental phosphorus to the aqueous phase. The reservoir is configured to provide an interfacial contact area between the liquid phases sufficient for the reaction. A catalyst bed is in contact with the water-immiscible liquid remote from the interface. The catalyst bed comprises a catalyst for the oxidation of elemental phosphorus by reaction with water.
The invention is further particularly directed to an apparatus comprising a reactant reservoir for an aqueous phase and a separate phase comprising elemental phosphorus. Means within the reservoir promotes mass transfer between the aqueous phase and the phase comprising elemental phosphorus. A fixed catalyst bed remote from the reservoir comprises a catalyst for the reaction. The apparatus further comprises means for circulating the phase comprising elemental phosphorus between the reservoir and the catalyst bed.
The invention is further directed to an apparatus for oxidation of elemental phosphorus to oxyacids of phosphorus comprising a catalyst slurry tank for a mixture of elemental phosphorus and catalyst for the oxidation of phosphorus by reaction with water, and a heterogeneous liquid phase reactor comprising a countercurrent liquid/liquid contact zone. The liquid phase reactor has an inlet for an aqueous liquid, an exit for an aqueous solution of phosphorus oxyacids, an inlet for a phosphorus phase, and an exit for a phosphorus phase. The apparatus further comprises means for circulating a phosphorus phase between the phosphorus phase exit of the liquid phase reactor, the catalyst slurry tank, and the phosphorus phase inlet of the reactor.
The invention is further directed to a composition effective for use in the manufacture of oxyacids of phosphorus. The composition comprises a mixture containing elemental phosphorus and a catalyst for the oxidation of elemental phosphorus by reaction with water.
The invention is also directed to apparatus for oxidation of elemental phosphorus to oxyacids of phosphorus comprising a reactor having a fixed catalyst bed positioned therein, said catalyst bed comprising a catalyst for the oxidation of elemental phosphorus to phosphorus oxyacids. The catalyst bed and a lift leg within the reactor and outside the catalyst bed are positioned within said reactor to provide access to the bottom of the lift leg by a phosphorus phase circulated from the bottom of the catalyst bed. The apparatus further comprises means for circulating aqueous liquid from an aqueous phase above the phosphorus phase in the reactor between an exit for the reactor in liquid flow communication with the upper end of the lift leg and a return to the reactor in liquid flow communication with the lower end of said lift leg within the phosphorus phase, whereby circulation of the aqueous liquid through the lift leg is effective to provide liquid/liquid contact between the phases and cause circulation of the phosphorus phase through the catalyst bed.
The invention is further directed to a process for the preparation of an oxyacid of phosphorus comprising oxidizing elemental phosphorus by catalytic reaction with water at a temperature below a threshold temperature at which the ratio of the sum of the rates of formation of P(I)+P(III) species to the rate of formation of P(V) species drops to 3.0 in a batch reaction system at 25% conversion of elemental phosphorus.
The invention is also directed to a process for the preparation of an oxyacid of phosphorus comprising catalytically oxidizing elemental phosphorus by reaction with water in a catalytic reaction zone comprising water, a phase containing elemental phosphorus and a catalyst for the reaction. Sonic and/or microwave energy is introduced into the reaction zone during the reaction.
Other objects and features will be in part apparent and in part pointed out hereinafter.